Born in London on 30 January 1909, after leaving Manchester Grammar School at 16, James Carreras managed the Oxford cinema, Manchester, and entered the distribution side of the industry in 1934, joining Exclusive Films, formed by his father, Enrique, and Will Hammer. He rose to Lieutenant-Colonel in World War II, and in 1946 returned as Managing Director of Exclusive, where he actually co-produced one film, Who Killed Van Loon? (d. Gordon Kyle, 1947), and oversaw the production of a steady stream of supporting films, mostly thrillers.

Hammer Films, established in a country house at Bray, grew out of Exclusive and began a series of highly profitable horror films from the mid-1950s, starting with The Quatermass Experiment (d. Val Guest, 1955).

A very astute man, with an instinct for popular entertainment, Carreras was described as 'managing director of the most successful privately-owned British film company'. He was the Chairman of Hammer Film Productions from 1949 to 1980, was made MBE in 1944 and knighted in 1970. His son was producer Michael Carreras.